Dichroic filters are a type of optical filters that sometimes are referred to as “reflective filters”, “thin film filters” or “interference filters”. Such dichroic filters can be made by coating a glass substrate with a series of optical coatings. These filters usually reflect an undesired portion of incident light and transmit the remainder of the incident light. A dichroic filter uses a principle of interference where layers of the dichroic filter form a sequential series of reflective cavities that resonate with the desired wave lengths, i.e. the remainder of the incident light that pass through the dichroic filter. Other wavelengths destructively cancel or reflect as the peaks and troughs of the waves overlap.
In a dichroic mirror or filter alternating layers of optical coatings with different refractive indexes are built up upon a glass substrate. The interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. By controlling the thickness and number of the layers, the frequency (wavelength) of the pass band of the filter can be tuned and made as wide or narrow as desired.
Several layer of different refractive index can be sandwiched together to obtain sharper cut of properties.
EAP (Electro Active Polymers) is a polymer material that can change size and shape in response to electrical stimulus.
There exists several types of EAP material, some of them are transparent.
An EAP material that has been used in industrial applications is Polyvinylidene fluoride (PVDF).
PVDF is a very thin plastic film with piezoelectric properties that is transparent.
One advantage with PVDF is the possibility to create very thin layer of this material.
Other piezoelectric plastic materials exist also.
The PVDF film can be coated with a conductive transparent material like Indium tin oxide (ITO), Carbon Nano Tubes (CNT) or Graphene. When a layer of CNT is thin enough, it is transparent.
The electric equivalent shows that EAP material is purely capacitive which means that power will only be consumed at transitions and not at a steady state.
There is a need for an optical filter that improves, in low light conditions, the sensitivity of the optical filter in the Infra red (IR) spectrum. It has then been suggested to use an IR filter in front of the camera with the “notch” filter characteristics showed in FIG. 1a. The filter can also be made to move the cut of wavelength towards higher wavelength, see FIG. 1b. 
The “notch” filter can block enough IR light to allow normal colour imaging while the “notch” filter also is transparent for an IR light range with a matched wavelength. This is a compromise when it comes to image quality. Normal daylight is rich in IR and a lot of undesired radiation may be incident on a sensor of a camera provided with such “notch” filter. This has a bad effect on colour reproduction.